Mutant glycoprotein resistant to modification with asparagine-linked sugar chain

ABSTRACT

To obtain a mutant protein of an asparagine-linked glycoprotein, which has no N-linked sugar chain under ordinary circumstance, and remains a physiological activity of the glycoprotein before the mutation was introduced, at least one of the amino acids contained in the amino acid sequence motif (I) and/or (II) in the polypeptide of the asparagine-linked glycoprotein is substituted into another amino acid: (I) Asn Xa1 Xa2 (II) Xa3 Val Gly Asn Xa1 Xa2. In amino acid sequence motif (I) and (II), Xa1 represents an amino acid other than Pro, Xa2 represents Thr or Ser and Xa3 represents His or Asp.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 11/658,103, filed Jan. 22, 2007, which is the U.S. National Phase under 35 U.S.C. §371 of International Application PCT/JP2005/013424, filed Jul. 21, 2005, which was published in a language other than English which claims priority of JP Application No. 2004-213616, filed Jul. 21, 2004.

TECHNICAL FIELD

The present invention relates to a mutant protein of an asparagine-linked glycoprotein, which has no N-linked sugar chain and retains a physiological activity of a glycoprotein before the mutation was introduced, and a method of producing the same.

BACKGROUND ART

First, abbreviations used in the specification are explained.

BPB: bromophenol blue

CMP: cytidine 5′-monophosphate

EDTA: ethylenediamine tetraacetic acid

EGTA: ethylene glycol bis(2-aminoethyl ether)-N,N,N′,N′-tetraacetic acid

Endo H: endo-β-N-acetylglucosaminidase H

ER: endoplasmic reticulum

FBS: fetal bovine serum

HRP: horseradish peroxidase

HPTLC: high performance thin-layer chromatography

LacCer: lactosylceramide

N-linked sugar chain: asparagine-linked sugar chain

N-linked glycoprotein: asparagine-linked glycoprotein

PBS: phosphate-buffered saline

PCR: polymerase chain reaction

PMSF: phenylmethanesulfonyl fluoride

PNGase F: peptide-N(4)-(N-acetyl-β-D-glucosaminyl) asparagine amidase

PVDF: polyvinylidene difluoride

SAT-I: sialyltransferase-I

hSAT-I: human SAT-I

mSAT-I: mouse SAT-I

zSAT-I: zebrafish SAT-I

SDS: sodium dodecyl sulfate

SDS-PAGE: sodium dodecyl sulfate-polyacrylamide electrophoresis

Further, in the specification, an amino acid X (one-letter notation) which is at the J-th position from the N-terminal of a protein is represented as “XJ”. For example, Asn at the 180th position from the N-terminal is represented as “N180” and Thr at the 336th position from the N-terminal is represented as “T336”.

Further, in the specification, a protein obtained by substituting an amino acid X (one-letter notation) at the J-th position from the N-terminal of a protein by an amino acid Z (one-letter notation) is represented as “XJZ”. For example, a protein in which His at the 177th position from the N-terminal is substituted by Asp is represented as “H 177D” and a protein in which Asn at the 224th position from the N-terminal is substituted by Lys is represented as “N224K”. Further, when substitution of one amino acid by another occurs at a plurality of sites, the substitutions are represented side by side. For example, a protein in which His at the 177th position from the N-terminal is substituted by Asp and Asn at the 224th position from the N-terminal is substituted by Lys is represented as “H177D, N224K”.

Most of secretory proteins in a living body are considered to be present as glycoproteins. Sugar chains have a wide variety of functions such as physical stabilization of proteins, expression of enzyme activity, cell adhesion, metastasis of cancer, signal transduction, subcellular localization, microbial infection, and immune response. Therefore, lack of the sugar chains to be linked to glycoproteins, in many cases, causes effects such as failure or reduction of expression of physiological activity of the glycoprotein.

An N-linked sugar chain, which is a sugar chain to be linked to a glycoprotein, is known to link to Asn in a consensus sequence comprising Asn-Xaa-Ser/Thr (provided that Xaa is an amino acid other than Pro) of a protein.

It is also known that a glycosyltransferase is one of glycoproteins, and a plurality of N-linked sugar chains are linked to most of glycosyltransferases. It has been reported that a glycosyltransferase wherein N in said consensus sequence is substituted by Q, which is structurally most similar to N, and a glycosyltransferase treated with tunicamycin, which inhibits formation of dolichol pyrophosphate-N-acetylglucosamine to suppress sugar transfer to a protein, have a remarkably reduced enzymatic activity (Non-patent Document 1 and Non-patent Document 2).

For example, α1,3-fucosyltransferase (Fuc) III, IV, V and VI, α2,3-sialyltransferase (ST3Gal I), α2,6-sialyltransferase (ST6Gal I), α2,8-sialyltransferase (ST8Sia I); GD3 synthase, UDP-N-acetylglucosamine:β-D-mannoside β1,4-N-acetylglucosaminyltransferase III (GnT III), core 2β1,6-N-acetylglucosaminyltransferase (C2 GnT), galactosylceramide sulfotransferase (CST), N-acetylgalactosaminyltransferase 1 (GalNac-T), β1,3-galactosyltransferase (Gal-T2), UDP-glucuronosyltransferase 2B (UGT2B) and the like are reported to require sugar chains to express their enzymatic activities.

SAT-I is one of sialyltransferases (i.e., sialic acid transferases) that transfer sialic acid to lactosylceramide and synthesizes GM3, and hSAT-I, mSAT-I and zSAT-I have been cloned (Non-patent Document 3 and Non-patent Document 4). In addition, SAT-Is from dog, bovine, rat, chicken, medaka, and tetradon have been cloned.

However, in N-linked glycoproteins, a mutant protein is not known which has no N-linked sugar chains to be linked thereto, but retains a physiological activity of a glycoprotein before the mutation is introduced. Neither is a method of producing such a mutant protein and the like.

-   Non-patent Document 1: Martina J. A. et al., 1998, The Journal of     Biological Chemistry, vol. 273, p. 3725-3731 -   Non-patent Document 2: Eckhardt M. et al., 2002, The Biochemical     Journal vol. 368, p. 317-324 -   Non-patent Document 3: Ishii A. et al., 1998, The Journal of     Biological Chemistry, vol. 273, p. 31652-31655 -   Non-patent Document 4: Kono M. et al., 1998, Biochemical and     Biophysical Research Communications, vol. 253, p. 170-175

SUMMARY OF THE INVENTION

If a physiological activity of a glycoprotein is expressed in a state where a sugar chain to be linked to the glycoprotein is absent, a protein which has the physiological activity equivalent to that of the glycoprotein can be produced conveniently, rapidly, in large amounts and at low cost and its quality can be kept constant. Therefore, it is an object of the present invention to provide a mutant protein of an N-linked glycoprotein, which lacks an N-linked sugar chain to be linked, but retains the physiological activity of the glycoprotein, and to provide a method of producing the same.

The inventors of the present invention made an intensive effort to achieve the above-mentioned objects and as a result, they have found that substitution of an amino acid in a specified amino acid sequence in the polypeptide of an N-linked glycoprotein by other amino acid allows the glycoprotein to retain the function of the glycoprotein before the mutation is introduced even in a state where N-linked sugar chains are absent, thereby accomplishing the present invention.

Thus, the present invention provides a mutant protein (hereinafter, referred to as “the protein of the present invention”) comprising an amino acid sequence of N-linked glycoprotein having the amino acid sequence motif (I) and/or (II) shown below, wherein at least one amino acid selected from the amino acids in the amino acid sequence motif (I) and/or (II) is substituted by other amino acid, and wherein the amino acid sequence containing the substituted amino acids is not subject to modification with N-linked sugar chain and an activity is maintained:

(I) Asn Xa1 Xa2 (SEQ ID NO: 1); and

(II) Xa3 Val Gly Asn Xa1 Xa2 (SEQ ID NO: 2)

(Xa1 indicates an amino acid other than Pro, Xa2 indicates Thr or Ser, and Xa3 indicates His or Asp in the amino acid sequence motif (I) and (II)).

Further, the amino acid substitution in the amino acid sequence motif (I) and/or (II) is preferably one or more substitutions (A) to (C) described below:

(A) substitution of Asn by Lys or Ser;

(B) substitution of Xa2 by Gln; and

(C) substitution of Xa3 by Asp.

Further, the glycoprotein is preferably an enzyme, more preferably a glycosyltransferase, in particular a sialic acid transferase.

Further, the sialyltransferase is preferably SAT-I, and in SAT-I, the amino acid sequence motif (I) is preferably an amino acid sequence (i) and/or (ii) described below, and the amino acid sequence motif (II) is preferably an amino acid sequence (iii) described below:

(i) Asn Glu Ser;

(ii) Asn Val Thr; and

(iii) His Val Gly Asn Lys Thr (SEQ ID NO: 43).

Further, the amino acid substitution in the amino acid sequences (i) to (iii) preferably includes one or more substitutions (a) to (d):

(a) substitution of Asn by Lys in the amino acid sequence (i);

(b) substitution of Thr by Gln in the amino acid sequence (ii);

(c) substitution of Asn by Ser in the amino acid sequence (iii); and

(d) substitution of His by Asp in the amino acid sequence (iii).

Further, the amino acid sequence of SAT-I before the mutation is introduced preferably has the amino acid sequence of SEQ ID NO: 4 or an amino acid sequence having homology of 80% or more with an amino acid sequence of SEQ ID NO: 4.

Further, the mutant SAT-1 is, in particular, preferably a protein having the following amino acid sequence.

-   -   the amino acid sequence of SEQ ID NO: 6.     -   the amino acid sequence of SEQ ID NO: 8.     -   the amino acid sequence of SEQ ID NO: 10.     -   the amino acid sequence of SEQ ID NO: 12.     -   the amino acid sequence of SEQ ID NO: 14.     -   the amino acid sequence of SEQ ID NO: 16.     -   the amino acid sequence of SEQ ID NO: 18.

Further, the present invention provides a polynucleotide encoding the protein of the present invention (hereinafter, referred to as “the polynucleotide of the present invention”).

Further, the present invention provides a method of producing a mutant protein that retains an activity (hereinafter, referred to as “the method of the present invention”), comprising:

comparing, in an amino acid sequence of an asparagine-linked glycoprotein having the amino acid sequence motif (I) and/or (II) shown below, the amino acid sequence motif (I) and/or (II) described below or an amino acid sequence comprising the amino acid sequence motif (I) and/or (II) with a corresponding protein from another organism, or with a corresponding amino acid sequence of another protein which is from the same organism as the glycoprotein and which belongs to the same protein family as the glycoprotein to clarify differences between the amino acid sequences; and

substituting the different amino acids by corresponding amino acids of the target of comparison, wherein the amino acid sequence comprising the substituted amino acids undergoes no modification by an asparagine-linked sugar chain:

(I) Asn Xa1 Xa2 (SEQ ID NO: 1); and

(II) Xa3 Val Gly Asn Xa1 Xa2 (SEQ ID NO: 2)

(Xa1 indicates an amino acid other than Pro, Xa2 indicates Thr or Ser, and Xa3 indicates His or Asp in the amino acid sequence motif (I) and (II)).

Further, in the method of the present invention, the amino acid substitution in the amino acid sequence motif (I) and/or (II) is preferably one or more substitutions (A) to (C):

(A) substitution of Asn by Lys or Ser;

(B) substitution of Xa2 by Gln; and

(C) substitution of Xa3 by Asp.

Further, the method of the present invention is preferably applied to an enzyme, more preferably to a glycosyltransferase, in particular to a sialic acid transferase.

The method of the present invention is preferably adapted to SAT-I among sialic acid transferases and in the SAT-I, the amino acid sequence motif (I) is preferably an amino acid sequence (i) and/or (ii) described below, and the amino acid sequence motif (II) is preferably an amino acid sequence (iii) described below:

(i) Asn Glu Ser;

(ii) Asn Val Thr; and

(iii) His Val Gly Asn Lys Thr (SEQ ID NO: 43).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing a comparison among amino acid sequences of hSAT-I, mSAT-I, and zSAT-I. In FIG. 1, the dotted crossbar line portion indicates a transmembrane region, the first solid crossbar line portion indicates a sialyl motif L, the second solid crossbar line portion indicates a sialyl motif S, and the third solid crossbar line portion indicates a sialyl motif VS. A boxed portion indicates the portion to which an N-linked sugar chain can be linked (i.e., N-glycosylation site).

FIG. 2 is a diagram (photograph) showing immunoblotting of cell lysates expressing mSAT-I.

FIG. 3 is a diagram (photograph) showing immunoblotting of cell lysates expressing mSAT-I in the presence of tunicamycin (Tuni.), Kifnecin (Kif.), or castanospermine (Cas.).

FIG. 4 is a diagram showing the SAT-I activity of the lysate used in FIG. 3. “*” indicates a significant difference with respect to the wild type at p<0.001.

FIG. 5 is a diagram (photograph) showing immunoblotting of cell lysates of expressing the mSAT-I mutant.

FIG. 6 is a diagram (photograph) showing immunoblotting of Endo H-treated cell lysates expressing the mSAT-I mutant.

FIG. 7 is a diagram (photograph) showing immunoblotting of PNGase F-treated cell lysates expressing the mSAT-I mutant.

FIG. 8 is a diagram showing results of FIG. 7 (i.e., total amount of SAT-I) in numerical values. “*” indicates a significant difference with respect to the wild type at p<0.05.

FIG. 9 is a diagram (photograph) showing sensitivity of the mSAT-I mutant to trypsin.

FIG. 10 is a diagram (photograph) showing the enzyme activity of the mSAT-I mutant. “*” indicates a significant difference with respect to the wild type at p<0.001.

FIG. 11 is a diagram (photograph) showing immunoblotting of cell lysates of expressing mSAT-I mutants.

FIG. 12 is a diagram showing the enzyme activity of the mSAT-1 mutants. “*” indicates a significant difference with respect to the wild type at p<0.001.

FIG. 13 is a diagram (photograph) showing immunoblotting of cell lysates expressing mSAT-1 mutants.

FIG. 14 is a diagram showing the enzyme activity of the mSAT-1 mutants. “*” indicates a significant difference with respect to the wild type at p<0.001 and “*” indicates a significant difference with respect to the wild type at p<0.05.

FIG. 15 is a diagram (photograph) showing immunoblotting of cell lysates of expressing mSAT-1 mutants.

FIG. 16 is a diagram showing the enzyme activity of the mSAT-1 mutants. “*” indicates a significant difference with respect to the wild type at p<0.001 and “*” indicates a significant difference with respect to the wild type at p<0.05.

FIG. 17 is a diagram (photograph) showing immunoblotting of cell lysates expressing mSAT-1 mutants.

FIG. 18 is a diagram showing the enzyme activity of the mSAT-1 mutant. “*” indicates a significant difference with respect to the wild type at p<0.001.

FIG. 19 is a diagram (photograph) showing immunoblotting of a lysate and culture medium of HEK293 cells which express mSAT-I. “1” to “4” indicate vectors used as controls, “5” to “8” show results of mSAT-I, “C” and “M” show results of a lysate of each cell and culture medium, respectively, PNGase F(−) indicates results without PNGase F treatment, and PNGase F(+) indicates results with PNGase F treatment.

FIG. 20 is a diagram showing the secretion amount of the mSAT-I mutant. Note that the secretion amount is indicated as a ratio of the amount of SAT-I protein secreted to a substrate to the amount of SAT-I protein present in the cell. “*” indicates a significant difference with respect to the wild type at p<0.001 and “**” indicates a significant difference with respect to the wild type at p<0.02.

FIG. 21 is a diagram (photograph) showing the enzyme activities of mSAT-I, “H177D, N180S, N224K, T336Q” mutants and the wild type mSAT-I which are expressed in E. coli. “-” in the first lane from the left indicates buffer only, and the first and second lanes from the right indicate 0.05 μg and 0.25 μg of GM3 standard, respectively.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, detailed description will be made with reference to embodiments of the present invention.

<1> Protein of the Present Invention

The protein of the present invention is a mutant protein comprising an amino acid sequence of N-linked glycoprotein having the amino acid sequence motif (I) and/or (II) described below, wherein at least one amino acid selected from the amino acid sequence motif (I) and/or (II) described below is substituted by other amino acid, and wherein the amino acid sequence comprising the substituted amino acids retains an activity without undergoing any modification by N-linked sugar chain:

(I) Asn Xa1 Xa2 (SEQ ID NO: 1); and

(II) Xa3 Val Gly Asn Xa1 Xa2 (SEQ ID NO: 2)

(in the amino acid sequence motif (I) and (II), Xa1 indicates an amino acid other than Pro, Xa2 indicates Thr or Ser, and Xa3 indicates His or Asp).

Herein, an N-linked glycoprotein into which the mutation is to be introduced is not particularly limited as far as it is a glycoprotein which contains an N-linked sugar chain. The N-linked sugar chains may be any one of high-mannose-type, complex-type, and hybrid-type and the number of the N-linked sugar chains is not limited. Further, the N-linked glycoprotein may contain an O-linked sugar chain.

Generally, in a polypeptide of a wild type N-linked glycoprotein, there is one or more amino acid sequences of the above-mentioned (I), (II) or both.

When the amino acid sequence motif (I) shown above is present at only one site in the glycoprotein into which the mutation is to be introduced, the protein of the present invention may be one in which at least one amino acid in the amino acid sequence motif is substituted. When the amino acid sequence motif (I) is present at two or more sites in the glycoprotein, the protein of the present invention may be one in which at least one amino acid is substituted in at least one of the amino acid sequences. Of course, the protein of the present invention may be one in which amino acids are substituted in all the amino acid sequence motifs that are present in the glycoprotein. The same applies to the amino acid sequence motif (II).

Further, when both the amino acid sequences (I) and (II) are present in the glycoprotein into which the mutation is introduced, the protein of the present invention may be one in which at least one amino acid is substituted in at least one of the amino acid sequences. Of course, the protein of the present invention may be one in which amino acids are substituted in both of them.

Therefore, when the amino acid sequence motif (I) is present at two sites and the amino acid sequence motif (II) is present at three sites in the glycoprotein, the protein of the present invention may be one in which one amino acid is substituted in at least one site among the five sites. In this case, the proteins of the present invention include, for example, one in which amino acids are substituted at two sites of the amino acid sequence motif (I) and at two sites of the amino acid sequence motif (II) among the five sites, and one in which amino acids are substituted at all the five sites.

In the proteins of the present invention obtained by amino acid substitution, Asn in the amino acid sequences (I) and (II) which contain the substituted amino acid does not undergo sugar chain modification. Since the N-linked sugar chain is linked to Asn in the consensus sequence comprising Asn-Xaa-Ser/Thr (Xaa is an amino acid other than Pro) in a protein, if the consensus sequence is destroyed by the above-mentioned substitution, the N-linked sugar chain is not retained in the above-mentioned amino acid sequences which contain the substituted amino acid. Although the mutant protein of the present invention may be one that has an N-linked sugar chain at site other than the substitution site, preferably it has no N-linked sugar chain as a whole.

A mutant protein in which at least one amino acid in the amino acid sequence (I) and/or (II) of the N-linked glycoprotein is substituted and which retains an activity can be obtained, for example: by comparing a glycosylation site or an amino acid sequence containing the glycosylation site with the amino acid sequence of a corresponding glycoprotein which is from another organism or which is from the same organism and belongs to the same protein family to clarify differences in the amino acid sequences at the glycosylation site between the protein in which a sugar chain is linked to the glycosylation site and the protein in which no sugar chain is linked to the glycosylation site; and by substituting the different amino acids in the protein in which a sugar chain is linked to the glycosylation site by the corresponding amino acids in the protein in which no sugar chain is linked to the glycosylation site.

In particular, the amino acid substitution in the amino acid sequence motif (I) and/or (II) is preferably one or more substitutions (A) to (C):

(A) substitution of Asn by Lys or Ser;

(B) substitution of Xa2 by Gln; and

(C) substitution of Xa3 by Asp.

For example, when the amino acid sequence motif (I) is present only at one site in a glycoprotein into which the mutation is introduced, the protein of the present invention may be any one of a protein in which Asn in the amino acid sequence is substituted by Lys or Ser, a protein in which Xa2 is substituted by Gln, or a protein in which both Asn and Xa2 are substituted. When the amino acid sequence motif (I) is present at 2 or more sites, the protein of the present invention may be one in which Asn in at least one of the amino acid sequences is substituted by Ser or Lys, or Xa2 in at least one of the amino acid sequences is substituted by Gln. Therefore, for example, when the amino acid sequence motif (I) is present at 6 sites in the glycoprotein into which the mutation is introduced, the protein of the present invention is one in which at least one of the substitutions selected from the substitution of Asn by Lys or Ser and the substitution of Xa2 by Gln occurs at least one site among the 6 sites.

When a plurality of the amino acid sequence motifs (I) are present in the glycoprotein into which the mutation is introduced, the kind and the number of amino acid substitutions of the protein of the present invention are not particularly limited.

That is, when the amino acid sequence motif (I) is present at 6 sites in the glycoprotein, the protein of the present invention may be one in which Asn is substituted by Ser in one of the amino acid sequences, Asn is substituted by Lys in another amino acid sequence, Xa2 is substituted by Gln in two other amino acid sequences, and the remaining two amino acid sequences are not substituted. Similar explanation is applicable to the amino acid motif (II).

In the present invention, the glycoprotein into which a mutation is introduced is preferably one which has some physiological activities. The protein of the present invention obtained by the above-mentioned amino acid substitution is characterized by retaining the physiological activity of the original glycoprotein even if it does not retain a part of or all the N-linked sugar chains to be linked. Whether the physiological activity of the glycoprotein is retained can be examined by comparing it with the physiological activity of the glycoprotein before the mutation is introduced by using a method of measuring the physiological activity that is selected as appropriate by one skilled in the art depending on the kind of the glycoprotein.

The phase “retains a physiological activity” does not necessarily mean that the mutant glycoprotein must have the same or higher activity than that of the N-linked glycoprotein before the mutation is introduced, as far as the mutant glycoprotein has the same kind of a physiological activity as the N-linked glycoprotein before the mutation is introduced. However, it is preferable that the mutant protein have preferably 30% or more activity, more preferably 50% or more activity than that of the N-linked glycoprotein before the mutation is introduced.

Further, examples of the glycoprotein having a physiological activity include enzymes, antibodies, cytokines, hormones, signal transducers, and receptors. These are exemplary and the present invention is not limited thereto. Among these, enzymes are preferable. Examples of the enzymes include glycosyltransferases, sulfotransferases, glycolytic enzymes, oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. These are exemplary and the present invention is not limited thereto. Among these, glycosyltransferases are preferable. Examples of the glycosyltransferases include sialyltransferase, fucosyltransferase, N-acetylglucosaminyltransferase, N-acetylgalactosaminyltransferase, galactosyltransferase, and glucuronosyltransferase. These are exemplary and the present invention is not limited thereto. Among these, sialic acid transferase is preferable.

The sialic acid transferase is not particularly limited and examples thereof include SAT-I, α2,3-sialyltransferase, α2,6-sialyltransferase, and α2,8-sialyltransferase, and SAT-1 is preferable.

In addition, in the amino acid sequence of SAT-I, the amino acid sequence motif (I) above is preferably the amino acid sequence (i) and/or (ii) described below, and the amino acid sequence motif (II) above is preferably the amino acid sequence of (iii) described below.

(i) Asn Glu Ser (amino acid sequence represented by the amino acid Nos. 224 to 226 of SEQ ID NO: 4);

(ii) Asn Val Thr (amino acid sequence represented by the amino acid Nos. 334 to 336 of SEQ ID NO: 4); and

(iii) His Val Gly Asn Lys Thr (SEQ ID NO: 43 and amino acid sequence represented by the amino acid Nos. 177 to 182 of SEQ ID NO: 4).

Further, the amino acid substitution in the amino acid sequences (i) to (iii) is preferably one or more substitutions (a) to (d):

(a) substitution of Asn by Lys in the amino acid sequence (i);

(b) substitution of Thr by Gln in the amino acid sequence (ii);

(c) substitution of Asn by Ser in the amino acid sequence (iii); and

(d) substitution of His by Asp in the amino acid sequence (iii).

The kind of the amino acid substitution is not particularly limited as far as the mutant SAT-I retains the enzymatic activity, but is preferably substitutions of any one of the above-mentioned items (a), (b), (c), and (d), a combination of the above-mentioned items (a) and (b), a combination of the above-mentioned items (c) and (d), a combination of the above-mentioned items (a), (b) and (c), or a combination of all of the above-mentioned items (a) to (d).

The amino acid sequence of SAT-I before the mutation is introduced is not particularly limited as far as it serves as an enzyme that transfers sialic acid to lactosylceramide to synthesize GM3, that is a sialyltransferase that catalyzes a reaction wherein sialic acid is transferred to non-reducing terminal galactose through α-2,3-linkage. The amino acid sequence of mSAT-I (SEQ ID NO: 4) is preferably used. Also, SAT-I that has an amino acid sequence having homology of 80% or more, preferably 90% or more, and more preferably 95% or more with the amino acid sequence of mSAT-I may be used.

It is also preferable to use the amino acid sequence of hSAT-I (SEQ ID NO: 42) as the amino acid sequence of SAT-I before the mutation is introduced. Also, SAT-I that has an amino acid sequence having homology of 80% or more, preferably 90% or more, and more preferably 95% or more with the amino acid sequence of hSAT-I may be used.

Further, the mutant SAT-1 is particularly preferably the protein having the following amino acid sequences.

-   -   the amino acid sequence of SEQ ID NO: 6.     -   the amino acid sequence of SEQ ID NO: 8.     -   the amino acid sequence of SEQ ID NO: 10.     -   the amino acid sequence of SEQ ID NO: 12.     -   the amino acid sequence of SEQ ID NO: 14.     -   the amino acid sequence of SEQ ID NO: 16.     -   the amino acid sequence of SEQ ID NO: 18.

In addition, other proteins (including peptide or peptides) may be linked to the protein of the present invention as far as the proteins give no substantial influence to the physiological activity of the protein of the present invention. That is, the term “protein of the present invention” includes a fusion protein composed of the protein of the present invention to which one or more other proteins (including peptide or peptides) is/are linked. The term “other proteins” as used herein includes peptides, and the kind of the other protein is not particularly limited and may be selected as appropriate depending on the purpose. Production of a fusion protein is intended to, for example, allow the protein to be secreted outside the cells, facilitate separation and purification, facilitate detection, or combine activities of a plurality of proteins. However, the object is not limited thereto. Reference is made to the explanation regarding the section “Polynucleotide of the Present Invention” described hereinbelow about examples of the term “other proteins”.

The protein of the present invention can be either chemically synthesized or produced by genetic engineering since it has been clarified by the present invention which part of it should be substituted and how it should be made. Regarding the method of producing the protein of the present invention by genetic engineering, reference is made to the section “Polynucleotide of the Present Invention” and Examples described hereinbelow.

Whether the protein of the present invention is obtained, synthesized, produced or the like can be readily judged by analyzing the amino acid sequence (or nucleotide sequence of the polynucleotide encoding the same) of a resultant protein and comparing the amino acid sequence with the sequence of the protein of the present invention (or the polynucleotide of the present invention).

Further, in the protein of the present invention, whether an N-linked sugar chain is linked to the amino acid sequence that contains the substituted amino acids can be conveniently examined by observing whether a sugar chain is released and whether the molecular weight of the protein is changed by allowing a suitable glycosidase (for example, PNGase F) to react with the protein.

<2> Polynucleotide of the Present Invention

The polynucleotide of the present invention is a polynucleotide that encodes the protein of the present invention.

The polynucleotide that encodes the protein of the present invention includes various polynucleotides having different nucleotide sequences due to degeneration of a genetic code. One skilled in the art can easily understand that any of such polynucleotides is included in the polynucleotide of the present invention.

For example, to obtain the mutant mSAT-I having amino acid sequences set forth below by introducing a mutation into mSAT-I having the amino acid sequence of SEQ ID NO: 4, the polynucleotides having the following nucleotide sequences may be used. However, it can be readily understood that these polynucleotides are exemplary and the present invention is not limited thereto.

(Protein)

-   -   the amino acid sequence of SEQ ID NO: 6.     -   the amino acid sequence of SEQ ID NO: 8.     -   the amino acid sequence of SEQ ID NO: 10.     -   the amino acid sequence of SEQ ID NO: 12.     -   the amino acid sequence of SEQ ID NO: 14.     -   the amino acid sequence of SEQ ID NO: 16.     -   the amino acid sequence of SEQ ID NO: 18.

(Polynucleotide)

-   -   the nucleotide sequence of SEQ ID NO: 5.     -   the nucleotide sequence of SEQ ID NO: 7.     -   the nucleotide sequence of SEQ ID NO: 9.     -   the nucleotide sequence of SEQ ID NO: 11.     -   the nucleotide sequence of SEQ ID NO: 13.     -   the nucleotide sequence of SEQ ID NO: 15.     -   the nucleotide sequence of SEQ ID NO: 17.

Although the nucleotides that constitute the polynucleotide of the present invention may be either a DNA or an RNA, it is preferably a DNA in terms of stability.

The polynucleotides of the present invention includes not only polynucleotides which directly encode the protein of the present invention but also polynucleotides having a sequence complementary to them.

The polynucleotide of the present invention can be produced, for example, by the following method.

First, the amino acid sequence of N-linked glycoprotein of interest is analyzed. Also, the nucleotide sequence of a polynucleotide that encodes the N-linked glycoprotein is determined as necessary. Then, nucleotides of a corresponding portion of the polynucleotide that encodes the glycoprotein is substituted by other nucleotides so that at least one amino acid of one or more of the amino acid sequence motif (I) and/or (II) present in the amino acid sequence is substituted by other amino acid(s):

(I) Asn Xa1 Xa2 (SEQ ID NO: 1); and

(II) Xa3 Val Gly Asn Xa1 Xa2 (SEQ ID NO: 2)

(in (I) and (II), Xa1 indicates an amino acid other than Pro, Xa2 indicates Thr or Ser, and Xa3 indicates His or Asp).

The number and kind of the nucleotide to be substituted are not particularly limited as far as the amino acid to be substituted can be substituted by other amino acid as a target. For example, there may be a case where substitution of only one nucleotide is sufficient or where three nucleotides must be substituted to effect the substitution by the target amino acid. How to introduce substitution of nucleotides can be determined as appropriate by one skilled in the art.

Since it has now been clarified what a portion and how a substitution should be performed, the substitution of nucleotides (i.e., production of the polynucleotide of the present invention) can be performed by using either a technique of chemical synthesis or a technique of genetic engineering.

In the case where the technique of chemical synthesis is used, chemical synthesis may be performed by designing a nucleotide sequence such that the substitution of interest can be introduced and then by performing chemical synthesis so that a nucleotide sequence as designed can be obtained.

In the case where the technique of genetic engineering is used, various methods can be adapted. For example, there can be used a method in which a primer corresponding to a portion of the polynucleotide encoding a target N-linked glycoprotein into which a nucleotide substitution is intended to be introduced (i.e., a primer having a nucleotide substituted by the target nucleotide) is synthesized, and a nucleotide is substituted using a polynucleotide encoding the target glycoprotein as a template and a commercially available mutation introducing kit (for example, QuickChange site-directed mutagenesis kit manufactured by Stratagene). For specific examples thereof, reference is made to the Examples below.

The obtained polynucleotide of the present invention may be further amplified or purified. Also, it may be incorporated into suitable plasmids, vectors, and so on.

The polynucleotide of the present invention thus obtained can be used in the production of the proteins of the present invention.

For example, PCR is performed by using the polynucleotide (i.e., DNA) of the present invention as a template to amplify the polynucleotide of the present invention. Then, the amplified polynucleotide of the present invention is incorporated into a suitable expression vector or the like.

The vector to be used herein is not particularly limited and can be selected as appropriate by one skilled in the art depending on the kind of cells (i.e., host) into which a gene is introduced and so on. For example, when eukaryotic cells are used as host cells, an expression vector for eukaryotic cells can be selected, while when prokaryotic cells are used as host cells, an expression vector for prokaryotic cells can be selected.

In particular, the expression vector for eukaryotic cells are preferably used, and an expression vector for mammalian cells are more preferably used.

Further, the expression vector may be constructed so that the protein of the present invention encoded by the polynucleotide of the present invention can be isolated and purified easily. In particular, when the expression vector is constructed so that the protein of the present invention is expressed in a form of a “fusion protein” comprising the protein of the present invention linked to other protein (for example, a labeled peptide), the protein of the present invention can be isolated and purified easily.

Examples of such “other protein” include various peptides such as signal peptides (i.e. peptides consisting of 15 to 30 amino acid residues which present at N-terminals of many proteins and function inside cells for the selection of proteins in the intracellular membrane penetration mechanism: for example, OmpA, OmpT, Dsb and so on), protein kinase A, protein A (a protein having a molecular weight of about 42,000, which is a constituent of a cell wall of Staphylococcus aureus), glutathione S-transferase, His tag (a sequence of 6 to 10 histidine residues arranged in a row), myc tag (a sequence of 13 amino acids from cMyc protein), FLAG peptide (a marker for analysis consisting of 8 amino acid residues), T7 tag (consisting of first 11 amino acid residues of gene 10 protein), S tag (consisting of 15 amino acid residues from pancreatic RNase A), HSV tag, pelB (a sequence consisting of 22 amino acids of Escherichia coli outer membrane protein pelB), HA tag (consisting of 10 amino acid residues from haemagglutinin), Trx tag (thioredoxin sequence), CBP tag (calmodulin binding peptide), CBD tag (cellulose binding domain), CBR tag (collagen binding domain), β-lac/blu (β-lactamase), β-gal (β-galactosidase), luc (luciferase), HP-Thio (His-patch thioredoxin), HSP (heat shock peptide), Lny (laminin-γ-peptide), Fn (fibronectin partial peptide), GFP (Green fluorescent peptide), YFP (yellow fluorescent peptide), CFP (cyan fluorescent peptide), BFP (blue fluorescent peptide), DsRed and DsRed2 (red fluorescent peptides), MBP (maltose binding peptide), LacZ (lactose operator), IgG (immunoglobulin G), avidin, and protein G.

Expression vectors into which the protein of the present invention has been incorporated are extracted and purified and then introduced into host cells. Host cells can be selected as appropriate depending on the kind of the expression vector to be used and are not particularly limited. Host cells may be either eukaryotic cells (e.g., mammalian cells, yeast, insect cells, and so on) or prokaryotic cells (e.g., Escherichia coli, Bacillus subtilis, and so on).

For example, when an expression vector for mammalian cells is used, mammalian cells can be used as host cells. The kind of the mammalian cells referred to herein is not particularly limited and can be selected as appropriate in terms of the object, expression efficiency, or the like. For example, chinese hamster ovary derived cell line (i.e., CHO cell line) and so on may be used.

The expression vector into which the polynucleotide (DNA) of the present invention has been incorporated is introduced into such cells and cultivated by conventional methods. The conditions of culture may be selected as appropriate depending on the kind of vector/host cells, objects, and so on.

In this manner, by culturing the cells into which the polynucleotide of the present invention has been introduced and collecting the culture product, the protein of the present invention is produced. As appropriate, the protein of the present invention may be further purified by using known extraction and purification methods.

<3> Method of the Present Invention

The method of the present invention is characterized by:

comparing, in an amino acid sequence of an N-linked glycoprotein having the amino acid sequence motif (I) and/or (II) shown below, the amino acid sequence motif (I) and/or (II) described below or an amino acid sequence comprising the amino acid sequence motif (I) and/or (II) with a corresponding protein from another organism, or with a corresponding amino acid sequence of another protein which is from the same organism as the glycoprotein and which belongs to the same protein family as the glycoprotein to clarify differences between the amino acid sequences; and

substituting the different amino acids by corresponding amino acids of the target of comparison:

(I) Asn Xa1 Xa2 (SEQ ID NO: 1); and

(II) Xa3 Val Gly Asn Xa1 Xa2 (SEQ ID NO: 2)

(Xa1 indicates an amino acid other than Pro, Xa2 indicates Thr or Ser, and Xa3 indicates His or Asp in the amino acid sequence motif (I) and (II)).

Further, in the case of comparing amino acid sequences according to the method of the present invention, only the glycosylation sites represented by the amino acid sequences (I) and (II) may be compared, but usually it is preferable that amino acid sequences including those sequences are compared in a wider range. Accordingly, it becomes possible to define the amino acid sequence corresponding to the glycosylation site.

The term “corresponding protein from another organism” is not particularly limited as far as it is a protein that has a similar physiological activity in an organism which belongs to a different genus. However, it is preferable that the protein from said organism has no N-linked sugar chain in a part of or the whole glycosylation site thereof and retains the physiological activity of the N-linked glycoprotein before the mutation is introduced. In the method of producing the mutant protein of the present invention from a wild type glycoprotein from a mammal, for example, an amino acid sequence of a wild type glycoprotein from a mammalian is compared preferably with an amino acid sequence of a corresponding wild type glycoprotein from fishes such as medaka or tetradon.

Further, another protein which is from the same organism as the glycoprotein that is the target of amino acid substitution and which belongs to the same protein family is not particularly limited as far as the protein is from the same organism as the glycoprotein that is the target of amino acid substitution and the protein has a physiological activity or structure similar to that of the glycoprotein. However, it is preferable that the another protein has no N-linked sugar chain in a part of or the whole glycosylation site thereof and has a physiological activity similar to that of the N-linked glycoprotein before the mutation is introduced.

The meanings of all the other terms described in connection with the method of the present invention are the same as those used in connection with the protein of the present invention.

Thus, the amino acid substitution in the amino acid sequence motif (I) and/or (II) is preferably one or more substitutions (A) to (C):

(A) Substitution of Asn by Lys or Ser;

(B) Substitution of Xa2 by Gln; and

(C) Substitution of Xa3 by Asp.

Further, the method of the present invention can preferably be applied to enzymes and more preferably glycosyltransferases. In particular, the method of the present invention is preferably applicable to sialyltransferase, and more preferably SAT-I.

Further, in the amino acid sequence of SAT-I before the mutation is introduced, the amino acid sequence motif (I) is preferably the amino acid sequence (i) and/or (ii), and the amino acid sequence motif (II) is preferably the amino acid sequence (iii):

(i) Asn Glu Ser (an amino acid sequence represented by the amino acid Nos. 224 to 226 of SEQ ID NO: 4);

(ii) Asn Val Thr (an amino acid sequence represented by the amino acid Nos. 334 to 336 of SEQ ID NO: 4); and

(iii) His Val Gly Asn Lys Thr (SEQ ID NO: 43 and an amino acid sequence represented by the amino acid Nos. 177 to 182 of SEQ ID NO: 4).

Further, at least one amino acid substitution of the amino acid sequences (i) to (iii) described below is preferably one or more substitutions (a) to (d):

(a) Substitution of Asn by Lys in the amino acid sequence (i);

(b) Substitution of Thr by Gln in the amino acid sequence (ii);

(c) Substitution of Asn by Ser in the amino acid sequence (iii); and

(d) Substitution of His by Asp in the amino acid sequence (iii).

The substitution of an amino acid in the method of the present invention can be performed according to the method described in the explanation of “the protein of the present invention” and “the polynucleotide of the present invention”.

In the method of the present invention, whether the protein retains an N-linked sugar chain or whether it retains the physiological activity of the glycoprotein can be confirmed by the method described in the explanation of “the protein of the present invention”.

EXAMPLES

Hereinafter, the present invention is described in more detail by referring to examples in which SAT-I (a kind of N-linked glycoprotein) is used.

<1> Materials, Methods, and so on

The materials, methods, and so on used in the present invention are as follows.

LIPOFECTAMINE™ 2000 Reagent: manufactured by Life Technology, Inc.

NUTRIENT MIXTURE F-12 HAM (HAM): manufactured by SIGMA.

PVDF membrane (i.e., Immobilon): manufactured by MILLIPORE.

X-ray film (i.e., Medical X-ray film): manufactured by Kodak.

Endo H: manufactured by New England Biolabs.

PNGase F: manufactured by New England Biolabs.

Tunicamycin: manufactured by SIGMA.

Kifnecin: manufactured by CALBIOCHEM.

Castanospermine: manufactured by CALBIOCHEM.

CMP-sialic acid: manufactured by SIGMA.

LacCer: manufactured by Matreya, Inc.

SepPak Plus 18: manufactured by Waters Associates (Milford, Mass.)).

Silica gel HPTLC: manufactured by Merck.

Anti-SAT-I antibody: anti-SAT-I rabbit IgG antibody (i.e., a polyclonal antibody prepared by using 51 residues at C-terminal side of mSAT-I as an antigen).

5×SDS sample buffer: 62.5 mM Tris (pH 6.8), 2% SDS, 10% glycerol, 0.001% BPB, and 5% 2-mercaptoethanol.

Running buffer: 25 mM Tris (pH 8.9), 192 mM glycine, 0.1% SDS.

Transfer buffer: 25 mM Tris, 192 mM glycine, 20% methanol.

TBS-T buffer: 137 mM NaCl, 20 mM Tris (pH 7.5), 0.05% Tween 20.

Blocking buffer: 5% skim milk solution prepared by using TBS-T buffer.

Cell suspension solution: 5% glycerol, 15 mM sodium cacodylate, 0.1% Lubrol (manufactured by ICN Biochemicals Inc.), protease inhibitor mixture (Complete™ EDTA free): manufactured by Roche Applied Science.

4× Reaction mixture: 400 mM sodium cacodylate, 40 mM MgCl₂, 1.6% Triton X-100.

PBS: 137 mM NaCl, 2.68 mM KCl, 10 mM Na₂HPO₄, 1.76 mM KH₂PO₄.

IP buffer: 50 mM Tris (pH 7.4), 150 mM NaCl, 2 mM NaF, 1 mM EDTA, 1 mM EGTA, 1% Triton X-100, 1 mM PMSF, protease inhibitor mixture (Complete™ EDTA free): manufactured by Roche Applied Science.

Cell culture: CHO cells were cultured by using NUTRIENT MIXTURE F-12 HAM medium containing 10% (v/v) FBS, 100 units/ml penicillin, 100 ng/ml streptomycin, 9.4% (v/v) sodium hydrogen carbonate, and 100 mM L-glutamine under conditions of 37° C. and 5% CO₂. The cells were recovered by using 0.25% trypsin/1 mM EDTA solution by subculturing once for every 3 days.

Introduction of SAT-I gene into CHO cells

CHO cells were inoculated on a 6-well plate and cultivated until 90% to 95% confluent was achieved. Then, the cells were gently washed with a medium that contained neither antibiotics nor serum (i.e., HAM (−/−)), followed by addition of 2.25 ml of the same medium and 250 μl of FBS.

Then, a plasmid (2 μg; pcDNA3.1 Zeo(+) (Invitrogen) into which the mSAT-I gene was incorporated) and 10 μl of LIPOFECTAMINE™ 200 Reagent were mixed with 250 μl of HAM (−/−), respectively, and the resultants were left to stand at room temperature for 5 minutes. After that, the both were mixed with each other and the resultant was left to stand at room temperature for 20 minutes. Then the resultant was added to the cells, followed by culturing for 24 hours. In the case where an inhibitor such as tunicamycin was added, the medium was replaced 6 hours after the transfection and inhibitors were added thereto (final concentration: tunicamycin: 2.5 Kifnecin: 5 μg/ml, castanospermine: 1 mM) and the mixture was cultured for 18 hours.

SDS-PAGE

5% polyacrylamide gel as a concentration gel and 7.5% or 10% polyacrylamide gel as a separation gel were used. 5×SDS sample buffer was added in a volume ⅓ of that of the sample solution and the mixture was boiled at 37° C. for about 3 minutes to denature the sample to prepare a sample for electrophoresis. By using a mini-real slab gel electrophoresis apparatus (manufactured by Biocraft, Inc.), electrophoresis was performed at a constant current of 20 mA for about 80 minutes. As a molecular weight marker serving as an index for molecular weight, Prestained Protein Marker was used. The molecular weights of the proteins included in the molecular weight marker were 175, 83, 62, 47.5, 32.5, 25, 16.5, and 6.5 kDa.

Immunoblotting

After SDS-PAGE was completed, the gel and the activated PVDF membrane were each equilibrated with transfer buffer by shaking for about 5 minutes. By using a semi-dry type transfer apparatus (trade name: Trans-Blot SD, manufactured by Biorad, Inc.), the protein in the gel was transferred onto the PVDF membrane at 10 V for 30 minutes. The PVDF membrane was blocked by using a blocking buffer at room temperature for 1 hour. After that, a primary antibody (i.e., anti-SAT-I antibody) was diluted 1,000 fold with the blocking buffer. The PVDF membrane was dipped in the resultant solution and shaken at overnight 4° C. After that, the PVDF membrane was dipped in a secondary antibody solution (i.e., anti-rabbit IgG labeled with HRP) diluted about 5,000 folds with TBS-T buffer, and allowed to react at room temperature for 1 hour.

The PVDF membrane after the reaction was colored with a Western blotting detecting reagent (i.e., ECL or ECL Plus (manufactured by Amersham Biosciences) or Lumi-Light Plus (manufactured by Roche Diagnostics Co., Ltd.). For detection, an X-ray film was used.

Sugar Chain Cleaving Reaction

Endo H or PNGase F was added to samples prepared by the alkali-acetone method and the samples were allowed to react at 37° C. for 1 hour. Then, the samples were analyzed by immunoblotting.

Assay of SAT-I Activity

CHO cells into which the mSAT-I gene was introduced (placed on a 6-well plate) were washed with ice-cold PBS and then 80 μl of solution for cell suspension was added to the cells and the cells were collected. After the cells were sonicated (once for 10 seconds), the sonicated product was centrifuged at 4° C. and 15,000×g for 10 minutes to collect the supernatant, which was used as an enzyme source. Further, 10.68 μl of 1 mg/ml LacCer was dried at room temperature, to which 10 μl of 4× Reaction mixture was added. The resultant was sonicated to suspend LacCer. To the suspension were added 20 μl of the enzyme source and a 1:4 mixture of radiolabeled CMP-sialic acid (i.e., Cytidine 5′-monophosphate, [4,5,6,7,8,9-¹⁴C]sialic acid (manufactured by NEN Life Science Products)) and non-radiolabeled CMP-sialic acid were added and the resultant mixture was allowed to react at 37° C. for 2 hours.

A fraction that contained lipids was purified from the solution after the reaction by using Sep-Pak Plus 18 and the resultant was applied to an HPTLC plate. Lipids were separated with a developing solvent (chloroform/methanol/0.2% CaCl₂ (55/45/10)).

Creation of SAT-I Mutant

Various mutants of mSAT-I were created by using mSAT-I as a template and QuickChange site-directed mutagenesis kit (manufactured by Stratagene) according to the recommendations described in the manual attached to the kit. The primers used for creating the various mutants are shown below.

N180Q: (SEQ ID NO: 19) 5′-CTCTGAACACGTTGGGCAGAAAACTACTATAAGG-3′ (SEQ ID NO: 20) 5′-CCTTATAGTAGTTTTCTGCCCAACGTGTTCAGAG-3′ N180K: (SEQ ID NO: 21) 5′-CTCTGAACACGTTGGGAAGAAAACTACTATAAGG-3′ (SEQ ID NO: 22) 5′-CCTTATAGTAGTTTTCTTCCCAACGTGTTCAGAG-3′ N180S: (SEQ ID NO: 23) 5′-CTCTGAACACGTTGGGAGCAAAACTACTATAAGG-3′ (SEQ ID NO: 24) 5′-CCTTATAGTAGTTTTGCTCCCAACGTGTTCAGAG-3′ H177D, N180S: (SEQ ID NO: 25) 5′-GAGGGTTACTCTGAAGAGGTTGGGCAGAAAACTACTATAAGG-3′ (SEQ ID NO: 26) 5′-CCTTATAGTAGTTTTGCTCCCAACGTCTTCAGAGTAACCCTC-3′ H177D: (SEQ ID NO: 27) 5′-GAGGGTTACTCTGAAGAGGTTGGGCAGAAAACTAC-3′ (SEQ ID NO: 28) 5′-GTAGTTTTATTCCCAACGTCTTCAGAGTAACCCTC-3′ N224Q: (SEQ ID NO: 29) 5′-GCAATGGTAAAACAGGAAAGCCTGCCC-3′ (SEQ ID NO: 30) 5′-GGGCAGGCTTTCCTGTTTTACCATTGC-3′ N224K: (SEQ ID NO: 31) 5′-GCAATGGTAAAAAAGGAAAGCCTGCCC-3′ (SEQ ID NO: 32) 5′-GGGCAGGCTTTCCTTTTTTACCATTGC-3′ N224D: (SEQ ID NO: 33) 5′-GCTTCAAGCAATGGTAAAACAGGAAAGCCTGCCCTTTTG-3′ (SEQ ID NO: 34) 5′-CAAAAGGGCAGGCTTTCATCTTTTACCATTGCTTGAAGC-3′ N334Q: (SEQ ID NO: 35) 5′-CTGGCAGGTCATGCACCAGGTGACCACAGAGACCAAG-3′ (SEQ ID NO: 36) 5′-CTTGGTCTCTGTGGTCACCTGGTGCATGACCTGCCAG-3′ N334K: (SEQ ID NO: 37) 5′-CTGGCAGGTCATGCACAAGGTGACCACAGAGACCAAG-3′ (SEQ ID NO: 38) 5′-CTTGGTCTCTGTGGTCACCTTGTGCATGACCTGCCAG-3′ T336Q: (SEQ ID NO: 39) 5′-CAGGTCATGCACAAGGTGACCACAGAGACCAAGTTCCTC-3′ (SEQ ID NO: 40) 5′-GAGGAACTTGGTCTCTGTCTGCACATTGTGCATGACCTG-3′

Trypsin Sensitivity Test

After washing of CHO cells into which the mSAT-I gene was transfected (placed on a 6-well plate) with ice-cold PBS, 100 μl of IP buffer was added thereto and the cells were collected. The cells were sonicated (once for 10 seconds), and centrifuged at 4° C. and 15,000×g for 10 minutes. The supernatant was collected, to which trypsin was added such that the final concentration was 0, 10, 20, 50, or 100 ng/μl and the resultants were allowed to react at 37° C. for 30 minutes. After that, 10% SDS was added to a final concentration of 1% to stop the reaction.

<2> Results

(1) Interspecies Comparison of N-Glycosylation Sites

Cloned SAT-Is of human, mouse, rat, and zebra-fish each have a transmembrane region in the N-terminal side and have sequences that are conserved in sialic acid transferase, called “sialyl motif L”, “sialyl motif S”, and “sialyl motif VS”. The sialyl motif L is known to participate in binding to CMP-sialic acid, while functions of the other motifs have not been defined yet. The results of amino acid sequence comparison between hSAT-I (SEQ ID NO: 42), mSAT-I (SEQ ID NO: 4) and zSAT-I (SEQ ID NO: 41) are shown in Table 1.

Further, N-linked sugar chains are known to link to the Asn residue of the amino acid sequence referred to as “Asn-Xaa-Thr/Ser” present in a glycoprotein.

In FIG. 1, the dotted crossbar line portion indicates a transmembrane region, the first solid crossbar line portion indicates a sialyl motif L, the second solid crossbar line portion indicates a sialyl motif S, and the third solid crossbar line portion indicates a sialyl motif VS. Further, the portion boxed with a bold line indicates a portion to which an N-linked sugar chain can be linked (N-glycosylation site). It has been shown that N-linked sugar chains can be linked to N180, N224, and N334 of mSAT-I.

Comparison of the amino acid sequences at the N-glycolation site indicates that except for the site in the sialyl motif L, the amino acid sequences at the N-glycosylation site are not conserved in the species. This suggests two possibilities that the sugar chains other than the sialyl motif L are not important to SAT-I, or that the modified amino acid sequence alternates the function of the sugar chain.

The comparison of the amino acid sequence of mSAT-I with the amino acid sequences of cloned SAT-Is from bovine, dog, rat, chicken, medaka, and a tetradon was performed according to the procedures of Clustal W (Thompson, J. D., Higgins, D. G, and Gibson, T. J. (1994) Nucleic Acids Res. 22, 4673-4680), and BOXSHADE (Institute for Animal Health, Surrey, UK), and indicated that regarding the amino acid sequence portions of the bovine, dog, rat, and chicken corresponding to the amino acid sequences of the N-glycosylation sites (around N180, N224, and N334) of mSAT-I, percentages of amino acid conservation were high. On the other hand, in medaka and tetradon, percentages of amino acid conservation were relatively low and the amino acids corresponding to N180, N224, and N334 were different, that is, they were S, K, and D, respectively.

Similarly, comparison of the amino acid sequence of the human-derived sialyltransferase with those of various enzyme families indicated that differences were observed among the enzyme families, that, for example, N at the 180th position in the glycosylation site present in the sialyl motif L in SAT-I was S or T, and H at the 177th position was D.

Further, the glycosylation site and its number in various sialyltransferases are known to differ from each other among species or enzyme families; this fact is considered to reflect a change in functional regulation of glycosyltransferases in the process of evolution to higher organisms. That is, the difference suggests the possibility that substitution of a particular amino acid allows to create an enzyme that retains its activity without sugar chains.

(2) Study on the Presence or Absence of Sugar Chains Linked to Sat-I and Kinds Thereof.

An anti-SAT-I antibody (polyclonal antibody) was prepared by using 51 residues at C-terminal of mSAT-I as an antigen. mSAT-I was transiently expressed by using CHO cells, and a lysate of the cells was subjected to SDS-PAGE and then to immunoblotting with an anti-SAT-I antibody. A lysate of the cells treated with Endo H or PNGase F was also subjected to immunoblotting with an anti-SAT-I antibody. The results are shown in FIG. 2.

The molecular weight of mSAT-I estimated by the amino acid sequence of mSAT-I is about 40 kDa. However, a major band having a larger molecular weight than that of mSAT-I was detected near 42 kDa and a broad band was detected from 45 kDa to 48 kDa (Lane 2 in FIG. 2). The result suggested that some modification was made to mSAT-I.

Further, treatment with PNGase F resulted in all the mSAT-I bands aggregated to a molecular weight of 40 kDa (Lane 4 in FIG. 2), while treatment with Endo H did not lead to complete cleavage of sugar chains, so that an Endo H-resistant band was detected (Lane 3 in FIG. 2). These results indicate that a complex sugar chain modified by the Golgi apparatus is linked to mSAT-I.

(3) Influence of Sugar Chain on Sat-I Activity

When mSAT-I was transiently expressed by using CHO cells, the cells were treated with an inhibitor (tunicamycin, Kifnecin, or castanospermine) of sugar chain processing in ER. Treatment with tunicamycin inhibited dolicholpyrophosphate-N-acetylglucosamine formation to suppress the glycosyltransfer to the protein. Treatment with Kifnecin inhibited mannosidase, resulting in generation of mSAT-I having a high mannose type sugar chain immediately before the transportation to the Golgi apparatus, preventing the formation of mSAT-I having a complex type sugar chain. Treatment with castanospermine causes inhibition of glucosidases I and II to suppress the interaction with calnexin or calreticulin that interacts by recognizing monoglucosylated sugar chains.

Lysates of the cells treated with those inhibitors were prepared in the presence or absence of Endo H and the lysates were subjected to SDS-PAGE, followed by immunoblotting with an anti-SAT-I antibody. Results are shown in FIG. 3. Results of assay of SAT-I activity by using the lysates are shown in FIG. 4.

FIG. 3 indicates that treatment with tunicamycin completely suppressed the glycosyltransfer to SAT-I and that treatment with Kifnecin or castanospermine resulted in linking of only high mannose type sugar chain (cleaved by Endo H).

FIG. 4 indicates that the activity of SAT-I to which no sugar chain was added by the treatment with tunicamycin decreased remarkably. On the other hand, SAT-I that retained the high mannose type sugar chain obtained by the treatment with Kifnecin or castanospermine had an activity equivalent to that of mSAT-I subjected to treatment without the inhibitors. These results indicated that a high mannose type sugar chain is essential for the activity of SAT-I and interaction with calnexin or calreticulin is unnecessary in the process of usual SAT-I folding.

(4) Creation of “N180Q”, “N224Q”, “N334Q”, and “N180Q, N224Q, N334Q” Mutants

Mutants (i.e., “N180Q”, “N224Q”, “N334Q”, and “N180Q, N224Q, N334Q”) in which any one or all of Asns in three N-glycosylation sites (i.e., N180, N224, and N334) present in mSAT-I was substituted by Gln were created.

These mutants were transiently expressed in CHO cells and the cell lysates were subjected to SDS-PAGE and immunoblotting with an anti-SAT-I antibody was performed. Results are shown in FIG. 5.

As a result, a decrease in molecular weight was observed in all the mutant and the “N180Q, N224Q, N334Q” mutant had the same molecular weight as that of the wild type mSAT-I treated with PNGase.

Further, in the N334Q mutant and the “N180Q, N224Q, N334Q” mutant, the amount of SAT-I remarkably decreased. The above-mentioned results indicated that a sugar chain was added to all the three N-glycosylation sites in mSAT-I and no modification of sugar chain occurred in other sites.

Then, the lysates of the cells in which various mutants were respectively expressed were treated with Endo H (37° C., 1 hour), and the treated lysates were subjected to SDS-PAGE, followed by immunoblotting with an anti-SAT-I antibody. Results are shown in FIG. 6.

As a result, Endo H-resistant band specifically decreased in the N180Q mutant. The decrease in Endo H-resistant band in the N334Q mutant was thought to be non-specific thereto from the fact that the Endo H-sensitive band similarly decreased.

Then, the lysates of the cells in which various mutants were respectively expressed were treated with PNGase F (37° C., 1 hour), and the treated lysates were subjected to SDS-PAGE, followed by immunoblotting with an anti-SAT-I antibody, to compare the amount of total SAT-I. Results of immunoblotting are shown in FIG. 7 and results of numerical conversion of the amount (mean value of three immunoblotting results) based on the results in FIG. 7 are shown in FIG. 8.

As a result, in the mutant N334Q and the mutant “N180Q, N224Q, N334Q”, a decrease in amount of protein of about 50% was indicated as compared with the wild type. However, when the cells were treated with tunicamycin, no such decrease was found, and it is unlikely that the sugar chain participates in the decrease in an amount of SAT-I. Further, results of a pulse label experiment with [³⁵5]-methionine indicated that the decrease occurred at the level of translation of protein. Therefore, it is considered that since the secondary structure of the mRNA was remarkably changed by mutation, the translation efficiency to a protein decreased. As a result, the amount of the proteins of the mutant N334Q and the mutant “N180Q, N224Q, N334Q” decreased.

(5) Comparison of Trypsin Sensitivities Among Various Mutants

An increase in sensitivity to proteases such as trypsin due to a miss folding of a protein has been known. Therefore, whether the structures of the mutants “N 180Q”, “N224Q”, “N334Q”, and “N180Q, N224Q, N334Q” were significantly changed was examined by comparing sensitivities of the mutants to trypsin. That is, mSAT-I was transiently expressed by using CHO cells, and a lysate of the cells was prepared. The lysate were treated with 0, 10, 20, 50, or 100 ng/ml trypsin at 37° C. for 1 hour. Results are shown in FIG. 9.

As shown in FIG. 9, no significant difference was observed in the sensitivity to trypsin between the wild type and the mutants. Therefore, it was suggested that the possibility that lack of sugar chains would remarkably change the conformation of SAT-I was low.

(6) Influence of Sugar Chain on mSAT-I Activity

Each mutant was transiently expressed by using CHO cells and the cell lysates were used as an enzyme source and their enzyme activities were assayed. Results are shown in FIG. 10.

As a result, in all the mutants N180Q, N224Q, N334Q, and “N180Q, N224Q, N334Q”, the activities decreased remarkably as compared with the wild type. In particular, the mutant N180Q had an activity lower than those of the mutants N224Q and N334Q, and in the mutant “N180Q, N224Q, N334Q” in which no sugar chain was linked, almost all the enzyme activity was lost. The above-mentioned results indicated that the presence of each sugar chain was essential for the enzyme activity of mSAT-I.

(7) Retention of Function of N-Linked Sugar Chains by Amino Acid Substitution (Retention of Physiological Activity of Protein)

Whether the function of an N-linked sugar chain is retained by substituting an amino acid at a particular portion (whether the physiological activity of the protein is retained without N-linked sugar chains modification) was examined by using mSAT-I.

First, sugar chains which link to N224 were examined. Mutants N224K (SEQ ID NO: 6) and N224D were created and their activities were compared with those of the wild type and the mutant N224Q.

These mutants were transiently expressed by using CHO cells and the cell lysates were subjected to SDS-PAGE, and immunoblotting with an anti-SAT-I antibody was performed. Results are shown in FIG. 11. The results indicated that the mutants N224K and N224D showed band patterns similar to that of the mutant N224Q.

The results of assay of the SAT-I activity of each mutant are shown in FIG. 12. The results indicated that the mutant N224K showed an activity of the same level as that of the wild type. On the other hand, in the mutant N224D in which substitution was made by D (Asp) having an opposite charge to K (Lys), the activity decreased more than that of the mutant N224Q. These results indicated that the function of the sugar chain which links to N224 was retained by substituting N224 by K (Lys) having a positive charge. The results also indicated that by performing such a substitution of an amino acid, the physiological activity (the enzymatic activity of SAT-0 of the protein was retained without N-linked sugar chain modification.

Then, the sugar chain which links to N334 was examined. The mutants N334K and T336Q (SEQ ID NO: 8) were created and their activities were compared with those of the wild type and a mutant N334Q.

These mutants were transiently expressed by using CHO cells and the cell lysates were subjected to SDS-PAGE, and immunoblotting with an anti-SAT-I antibody was performed. Results are shown in FIG. 13. The results indicated that both the mutants N334K and T336Q showed band patterns similar to that of the mutant N334Q.

Further, results of assay of SAT-I activity of each mutant are shown in FIG. 14. The results indicated that the mutant N334K only showed an activity of the same level as that of the mutant N334Q while the mutant T336Q showed an activity about 1.5 folds higher than that of the wild type. These results indicated that the function of the sugar chain which links to N334 was retained by substituting T336 by Q (Gln). The results also indicated that by performing such a substitution of an amino acid, the physiological activity (the enzymatic activity of SAT-I) of the protein was retained without N-linked sugar chain modification.

Then, the sugar chain which links to N180 was examined. Mutants N180K, N180S (SEQ ID NO: 10), “H177D, N180S” (SEQ ID NO: 12), and H177D (SEQ ID NO: 14) were created and their activities were compared with those of the wild type and a mutant N180Q.

These mutants were transiently expressed by using CHO cells and the cell lysates were subjected to SDS-PAGE, and immunoblotting with an anti-SAT-I antibody was performed. Results are shown in FIG. 15. The results indicated that the mutants N 180S and “H177D, N180S” had a major band around 42 kDa similar to that of the mutant N180Q and the mutant N180K had major bands at around 42 kDa and 40 kDa. The mutant H177D showed a similar band pattern to that of the wild type.

Further, results of assay of SAT-I activity of each mutant are shown in FIG. 16. The results indicated that the mutant N180K only showed an activity of the same level as that of the mutant N180Q while the mutant N180S showed an activity of about 50% of that of the wild type, and the mutant “H177D, N180S” had an activity equivalent to or higher than that of the wild type. Further, the mutant H177D showed an activity of about 1.5 folds higher than that of the wild type. These results indicated that the function of the sugar chain which links to N180 was retained by substituting H177 by D (Asp) and/or substituting N180 by S (Ser). The results also indicated that by performing such a substitution of an amino acid, the physiological activity (the enzymatic activity of SAT-I) of the protein was retained without N-linked sugar chain modification.

Then, the mutant “N180S, N224K, T336Q” having no sugar chain modification site and the mutant “H177D, N180S, N224K, T336Q” (SEQ ID No: 16) were created and their activities were compared with those of the wild type and the mutant “N180Q, N224Q, N334Q”.

These mutants were transiently expressed by using CHO cells and the cell lysates were subjected to SDS-PAGE, and immunoblotting with an anti-SAT-I antibody was performed. Results are shown in FIG. 17. The results indicated that both mutants showed a band around 35 kDa in a manner similar to the case of PNGase F treatment.

Further, results of assay of SAT-I activity of each mutant are shown in FIG. 18. The results indicated that the mutant “N180S, N224K, T336Q” showed an activity of about 40% of that of the wild type, and the mutant “H177D, N180S, N224K, T336Q” showed an activity of about 50% of that of the wild type. Further, separately, the mutant “N224K, T336Q” (SEQ ID NO: 18) was created and the activity of SAT-I was assayed similarly. As a result, the mutant showed an activity equivalent to that of the wild type.

The results indicated that, in the amino acid sequence of N-linked glycoprotein having the amino acid sequence motif (I) and/or (II) described below, substitution of an amino acid in the amino acid sequence motif (I) and/or (II) by other amino acid(s) according to one or more of rules (A) to (C) described below allowed the physiological activity of the glycoprotein before the mutation is introduced to be retained without being subjected to modification by an N-linked sugar chain in the following amino acid sequence containing the amino acid to be substituted:

(I) Asn Xa1 Xa2 (SEQ ID NO: 1); and

(II) Xa3 Val Gly Asn Xa1 Xa2 (SEQ ID NO: 2)

(in the amino acid sequence motif (I) and (II), Xa1 indicates an amino acid other than Pro, Xa2 indicates Thr or Ser, and Xa3 indicates His or Asp);

(A) substitution of Asn by Lys or Ser;

(B) substitution of Xa2 by Gln; and

(C) substitution of Xa3 by Asp.

(8) Influence of Amino Acid Substitution on Extracellular Secretion

Experiments on the extracellular secretion of SAT-I were performed.

In a similar manner as the transfection using the above-described CHO cells, HEK293 cells were cultured in a Dulbecco modified Eagle medium and SAT-I was transiently expressed in the HEK293 cells by using a pcDNA3 vector having a wild type mSAT-I inserted therein (wild type pcDNA3.1 Zeo(+)-mST3Gal-V). After the culture, the cell lysates and the culture were separately collected and the lysate of the cell lysates and the culture were subjected to SDS-PAGE, and immunoblotting with an anti-SAT-I antibody was performed. Further, each of the lysate of the cells and the medium was treated with PNGase F and then immunoblotting was similarly performed, and the results thereof were compared (FIG. 19). As a control, the pcDNA3 vector was used.

SAT-I secreted in the medium showed broad bands of 45 kDa and 38 kDa. It was predicted that the 45 kDa band retains an N-linked sugar chain since the 45 kDa band was not detected in immunoblotting after the PNGase F treatment. The 45 kDa band was resistant to Endo H.

Further, by using the HEK293 cells, various mutants of mSAT-I created as described above (the mutants N180Q, “H177D, N180S”, “N224Q”, N224K, N334Q, and T336Q) and the wild type mSAT-I were transiently expressed, and mSAT-I extracted from the cell lysates and mSAT-I secreted in the culture were subjected to PNGase F treatment and then subjected to SDS-PAGE, and immunoblotting with an anti-SAT-I antibody was performed. The bands were analyzed by measuring concentrations thereof, and a ratio of the amount of protein secreted to the substrate to the amount of protein present in the cells was calculated. Further, setting the ratio of the wild type mSAT-I as 100%, the secretion amount of each mutant was calculated (FIG. 20).

The secretion amounts of the mutant N224Q and of the mutant N224K were substantially the same as that of the wild type. On the other hand, the mutant N180Q had a secretion amount of 20% of that of the wild type and the mutant “H177D, N180S” had a secretion amount of 50% of the wild type. Surprisingly, the mutant N334Q and the mutant T336Q had a secretion amount higher than 300% of the wild type. These results revealed that the N-linked sugar chain linked to the N at the 180th position in SAT-I played an important role in secretion into a medium and on the other hand, the N-linked sugar chain linked to the N at the 334th position participates in the function of retaining the protein in the cells.

The secretion amount of the mutant “H177D, N180S” which retained an activity substantially equivalent to that of the wild type was approximately two times the secretion amount of the mutant N180Q which showed an activity of only about 20% of the activity of the wild type.

The mutant N180Q had an abnormal structure that is enzymatically inactive due to loss of the N-linked sugar chain, so it had an activity of about 20% of the wild type and a secretion amount of about 20% of the wild type. On the other hand, the mutant “H177D, N180S” had an enzyme activity substantially equivalent to that of the wild type, so it was suggested that it retained the enzyme activity structure. It was predicted that the decrease in the secretion amount was due to the loss of the N-linked sugar chain linked to the N at the 180th position.

These results suggested that not only the N-linked sugar chain linked to N at the 180th position but also the protein structure near this site is involved in the activity and secretion of SAT-I.

When the protein of the present invention is produced by genetic engineering, regulation of the function relating to the extracellular secretion is important. The method of obtaining the target protein of the present invention from the culture medium is more convenient and efficient than the method of obtaining the target protein of the present invention from cultured cells.

(9) Expression of Mutant “H177D, N180S, N224K, T336Q” of mSAT-I in Escherichia coli

(i) Expression of Mutant mSAT-I in Escherichia coli

Expression of mutant mSAT-I in Escherichia coli was performed in a Cold Shock Expression System (manufactured by TaKaRa Bio Co., Ltd.).

Specifically, an Escherichia coli BL21 strain was transformed with Chaperon plasmid pG-Tf2 (manufactured by TaKaRa Bio Co., Ltd.) to create BL21-pG-Tf2 and then competent cells of the Escherichia coli BL21 strain were prepared. Then, a (ATM) mutant mSAT-I gene in which a transmembrane region (TM) was deleted was introduced into SAT-I expression plasmids (Psu141 and Psu142) and the competent cell was transformed again with these plasmids to create BL21-pG-Tf2-pSU141 and BL21-pG-Tf2-pSU142. The cells were preincubated in 1 ml of an LB medium (tryptone, yeast extract, NaCl, pH 7.2) at 37° C. and then the preincubated cells were added to the LB medium so that the preincubated cells are diluted such that optical density OD at 600 nm was 0.1. At the same time, tetracycline was added up to 5 ng/ml and the cells were cultured at 37° C. until optical density OD at 600 nm was 0.53. Then, the culture temperature was changed to 15° C. and culture was continued for additional 30 minutes. After that, IPTG (isopropyl (3-D-thiogalactopyranoside/manufactured by SIGMA) was added to the medium up to 0.01 mmol/l and the cells were cultured at 15° C. for 48 hours. After completion of the culture, cells were collected by centrifugation. The collected cells were suspended in PBS containing 1 ml of 1 mmol/l fluorinated 4-(2-aminoethyl)benzenesulfonyl hydrochloride (ABESF; protease inhibitor) and sonicated twice for 30 seconds, and then centrifuged at 20,000×g for 15 minutes to collect a supernatant.

As a control, expression of wild type ΔTM-mSAT-I was similarly performed.

(ii) Purification of Mutant mSAT-I Expressed in Escherichia coli

Then, purification of mutant mSAT-I was performed. First, Ni-NTA superflow (manufactured by QIAGEN) was aliquoted in a 1.5-ml tube so that the amount of gel was 10 μl, and 100 μl of PBS containing 0.01% Triton X-100 was added thereto and the gel was allowed to be mixed well. Then, the gel was subjected to equilibration treatment and centrifuged to remove a supernatant. Further, the supernatant obtained in the section (i) mentioned above was aliquoted and gently mixed with the gel by tapping and then the mixture was allowed to react at 4° C. for 2 hours in a rotary mixer. After the reaction, the reaction mixture was centrifuged to remove the supernatant and wash buffer (composition: 50 mM NaH₂PO₄ (pH 8.0), 300 mM NaCl, 20 mM imidazole, 0.01% Triton X-100) was added thereto. The resultant mixture was centrifuged again to remove a supernatant, and elute buffer (composition: 50 mM NaH₂PO₄ (pH 8.0), 300 mM NaCl, 125 mM imidazole) was added thereto. The resultant mixture was gently mixed by tapping and centrifuged again to collect the supernatant. The collected supernatant was used as an Ni-affinity-purified sample.

Similarly, wild type mSAT-I was purified.

Further, imidazole in the elute buffer mentioned above was dialyzed and substituted by 20 mM Tris hydrochloride (pH 7.2) and 150 mM NaCl and used as an enzyme source as described in the section (iii) below.

(iii) Assay of Activity of Mutant mSAT-I

After a solvent (chloroform:methanol=1:1) of LacCer solution was dried 4× Reaction mixture was added four times and LacCer was suspended by sonication. Further, only 50 μl of 3 mM CMP-sialic acid, 100 μl of an enzyme source, and buffer (composition: 20 mM Tris hydrochloride, 150 mM NaCl, pH 7.2) were added and the resultant was allowed to react at 37° C. for 2 hours. Ganglioside GM3 was purified from the reaction solution by means of Sep-Pak Plus18 and then was applied to an HPTLC plate. At the same time, a standard substance of GM3 (0.25 or 0.05 μg) was added. As a developing solvent, chloroform/methanol/0.2% CaCl₂ (55/45/10) was used to separate lipids. All the lipids were thermally transferred from the HPTLC plate to a PVDF membrane. The obtained PVDF membrane was dried and then blocked with blocking buffer to allow a primary antibody (M2590 which is an IgM monoclonal antibody specific to GM3 was used) to react with the membrane in the blocking buffer. After the PVDF membrane was well washed with TBS-T buffer, the PVDF membrane was allowed to react with a secondary antibody (anti-murine IgM antibody labeled with HRP) in the blocking buffer. The PVDF membrane was well washed with TBS-T buffer, and then the PVDF membrane was allowed to develop chemiluminescences with an ECL kit (manufactured by Amersham Biosciences) and GM3 was detected with an X-ray film.

Similarly, wild type mSAT-I that was expressed in Escherichia coli was also assayed for its activity.

From the results (FIG. 21), 0.05 μg or less of GM3 was detected in the case of the wild type mSAT-I, and about 0.5 μg of GM3 was detected in the case of the mutant SAT-I. Those results suggested that the mutant mSAT-I had an activity 10 folds or more as compared with that of the wild type SAT-I.

These results indicated that the gene of the mutant SAT-I which was not modified with sugar chains was expressed in Escherichia coli and had an activity equivalent to that of the wild type. The mutant protein of the present invention is a protein which has an activity without sugar chain modification, so the mutant protein can be mass-produced by using microbial cells such as Escherichia coli, and thus is useful.

The present invention also encompasses a concept of a method of allowing a physiological activity of a protein to be retained without sugar chain modification, comprising: comparing amino acid sequences at a particular glycosylation site in N-linked glcoprotein or neighborhood sites including the same among a plurality of animal species to clarify differences in amino acid sequence between the animal species in which a sugar chain is linked at the glycosylation site and the animal species in which no sugar chain is linked at the glycosylation site; and substituting different amino acids in the animal species in which a sugar chain is linked at the glycosylation site by the corresponding amino acids in the animal species in which no sugar chain is linked at the glycosylation site.

The protein of the present invention is an N-linked glycoprotein in which a sugar chain that participates in expression of various functions, which retains the physiological activity of the glycoprotein without modification of a part of or the whole of the N-linked sugar chains. Hence a protein that has a function equivalent to that of the N-linked glycoprotein before the mutation is introduced can be produced conveniently, rapidly, in large amounts, and at low cost and further the quality of the protein to be produced can be maintained at a constant level. Therefore, the protein of the present invention is extremely useful. By utilizing the polynucleotide of the present invention, production of the protein of the present invention can be performed more conveniently and more rapidly, so the polynucleotide of the present invention is extremely useful. Further, the method of the present invention is extremely useful because it allows physiological functions of glycoproteins to be expressed without sugar chain modification. As described above, the present invention is extremely useful as a tool for synthesis or the like of pharmaceuticals, reagents, and various substances.

INDUSTRIAL APPLICABILITY

The protein of the present invention has a function which is equivalent to that of the N-linked glycoprotein before the mutation is introduced, so it can be utilized as a tool for synthesis of pharmaceuticals, reagents, and various substances or the like. The polynucleotide of the present invention can be utilized as a tool for production of the protein of the present invention or the like. Further, the method of the present invention can be applied to the production of the protein of the present invention or the like. 

1. A mutant sialyltransferase-I (SAT-I) comprising the amino acid sequence of SEQ ID NO:4 into which at least one amino acid substitution selected from the group consisting of (a) to (d) shown below has been introduced: (a) substitution of Asn by Lys in the amino acid sequence motif (i) Asn Glu Ser (amino acids 224-226 of SEQ ID NO:4); (b) substitution of Thr by Gln in the amino acid sequence motif (ii) Asn Val Thr (amino acids 334-336 of SEQ ID NO:4); (c) substitution of Asn by Ser in the amino acid sequence motif (iii) His Val Gly Asn Lys Thr (SEQ ID NO: 43) (amino acids 177-182 of SEQ ID NO:4); and (d) substitution of His by Asp in the amino acid sequence motif (iii) His Val Gly Asn Lys Thr (SEQ ID NO: 43) (amino acids 177-182 of SEQ ID NO:4).
 2. The mutant SAT-I according to claim 1, which comprises the amino acid sequence of SEQ ID NO:
 6. 3. The mutant SAT-I according to claim 1, which comprises the amino acid sequence of SEQ ID NO:
 8. 4. The mutant SAT-I according to claim 1, which comprises the amino acid sequence of SEQ ID NO:
 10. 5. The mutant SAT-I according to claim 1, which comprises the amino acid sequence of SEQ ID NO:
 12. 6. The mutant SAT-I according to claim 1, which comprises the amino acid sequence of SEQ ID NO:
 14. 7. The mutant SAT-I according to claim 1, which comprises the amino acid sequence of SEQ ID NO:
 16. 8. The mutant SAT-I according to claim 1, which comprises the amino acid sequence of SEQ ID NO:
 18. 